The goal of this project is to identify small ligands which may be used to specifically deliver cytotoxic compounds to breast tumor cells. Ligands much smaller than antibody fragments may have important advantages in targeted cancer therapy including improvement of tumor to nontumor ratios of ligand binding, better penetration of solid tumors, and non-immunogenicity. This proposal describes the use of random peptide library technologies, which have revolutionized the drug discovery process, to identify small ligands to tumor-specific targets. These ligands can then be coupled to cytotoxic agents and used to mediate the specific destruction of tumor cells. Our primary target is the extracellular domain (ECD) of ErbB2, a promising breast cancer tumor-specific target found on the cell surface of the tumor cells of at least 30 percent of breast cancer patients and, importantly, associated with a poor prognosis. Using an ErbB2 dimerization assay we have developed, we will also attempt to identify peptides which inhibit dimerization. Such peptides may have therapeutic value, even without coupling to cytotoxic agents. Several consensus amino acid sequences which may bind to ErbB2 have been identified by extensive screening with a peptide library we constructed, although the affinity of these peptides was not high enough to allow unequivocal proof of their specificity. Peptide library technology has evolved rapidly, and it has become apparent that one library is not adequate to obtain high affinity binders to any given target. A large panel of libraries which present a vast number of peptides presented in a variety of structural contexts is much more likely to yield a high affinity binder. The Specific Aims of this project are to: (1) Construct a large panel of random peptide libraries (2) Identify and characterize displayed peptide binders to ErbB2 ECD (3) Synthesize and characterize free peptides which bind to ErbB2 ECD (4) Construct mutagenesis libraries and/or synthesize peptidomimetics using identified consensus sequences to increase the affinity and/or stability of peptide ligands (5) Identify and further characterize small ligands to other promising tumor targets. This project will result in the identification of tumor-specific ligands with the affinity and specificity of antibodies but with the more favorable pharmacokinetics of low molecular weight drugs. Specifically, this work will produce lead compounds for drugs with great potential efficacy in the treatment of breast cancer.